Boron nitride (BN) is isoelectronic to a similarly structured carbon lattice while including an equal number of boron and nitrogen atoms. Like carbon material, BN exists in various crystalline forms, such as amorphous (a-BN), hexagonal (h-BN), cubic (c-BN) and wurtzite (w-BN). Among all these different crystalline forms, the hexagonal form is the most stable one. Similar to graphite, the boron and nitrogen atoms in each layer of h-BN are bound together by strong covalent bonds, forming the hexagonal BN sheet, while between layers, bonding is via the weak van der Waals force.
Based on their unique two-dimensional planar structure, h-BN thin films have a wide range of attractive properties, including high-temperature stability, low dielectric constant, high strength, large thermal conductivity, high hardness and high corrosion resistance. The decomposition temperatures of h-BN are up to 1000° C. in air and 2800° C. in an inert atmosphere. h-BN, therefore, is a good lubricant even at extremely high temperatures. Compared to graphite, h-BN is also more useful when electrical insulation is required.
Besides its traditional usages, h-BN also has great potential in microelectronic devices and nanotechnology. Similar to aluminum nitride (AlN) and gallium nitride (GaN), h-BN is a wide-gap semiconductor with a band gap energy corresponding to the ultraviolet (UV) region (around 6 eV). Furthermore, h-BN is more chemically and thermally stable than AlN and GaN, making h-BN one of the most promising materials for UV optical devices, even working in extreme environments. When voltage is applied to h-BN, it can emit UV light in the range 215-250 nm and can therefore potentially be used as a light emitting diode (LED) or as a UV-laser. Boron nitride can also be p-doped by beryllium or n-doped by sulfur. H-BN, accordingly, has the potential to perform as a p-n junction device. Due to its excellent dielectric properties, h-BN can also be used in electronics, for example, as a substrate for semiconductors and microwave-transparent windows. h-BN is also used as a charge-leakage barrier layer of the photo drum in laser printers.
Although with such significant advantages and wide applications for h-BN, methods for obtaining h-BN, especially an h-BN thin film with an atomic smooth surface, are very limited. Chemical vapor deposition (CVD) has been used to obtain h-BN thin film for electronic applications, wherein various chemical systems, such as BCl3/NH3 and B2H6/NH3, have been employed for h-BN synthesis. Controlling the surface morphologies and B/N stoichiometries, however, is very challenging. Borazine (B3N3H6) is a good precursor for h-BN synthesis due to the 1:1 B/N stoichiometry. Monolayer h-BN thin film can be obtained by exposure of borazine to nickel (111) or to another transition metal surface, but the known method uses an ultrahigh-vacuum chamber with a high-temperature growth condition. Additionally, growth of more than one layer of h-BN via this method is far more difficult; therefore, a technique for growing h-BN thin film in a controlled manner is highly demanded.
Additional discussion relating to boron nitride is provided in the following references: U.S. Pat. No. 5,079,038; U.S. Pat. No. 3,321,337; US Published Application No. 2008/0038585 A1; US Published Application No. 2006/0286814 A1; US Published Application No. 2010/0021708 A1; P. Fazen, et al., “Synthesis, Properties, and Ceramic Conversion Reactions of Polyborazylene, A High-Yield Polymeric Precursor to Boron Nitride,” 7 Chem. Mater. 1942-1956 (1995); C. Oshima, et al., “A Heteroepitaxial Multi-Atomic-Layer System of Graphene and h-BN,” 7 Surface Review and Letters 521-525 (2000); A. Nagashima, et al., “Electronic Dispersion Relations of Monolayer Hexagonal Boron Nitride Formed on the Ni(111) Surface,” 51 Physical Review B 4606-4613 (1995); K. Watanabe, et al., “Far-Ultraviolet Plane-Emission Handheld Device Based on Hexagonal Boron Nitride,” 3 Nature Photonics 591-594 (20 Sep. 2009); K. Watanabe, et al., “Ultraviolet Luminescence Spectra of Boron Nitride Single Crystals Grown under High Pressure and High Temperature,” 201 Phys. Stat. Sol. (a) 2561-2565 (2004); K. Watanabe, et al. “Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal,” 3 Nature Materials 404-409 (2004).